It is known that the particulate filtration from hot gas can be performed by means of ceramic filter elements arranged in a filter vessel. The walls of the filter vessel are protected against high temperature by appropriate insulation. The filter vessel further usually comprises water cooled steel support plates for supporting the filter elements. The construction is thereby heavy and space consuming. The ratio of filtrate volume flow to filtration area of conventional ceramic filters is low and therefore makes the use of these filters economically unjustified especially for hot gas cleaning in atmospheric conditions.
Reduction of pollutant gaseous emissions can be done by means of the Selective Catalytic Reduction (SCR) method or Selective Noncatalytic Reduction (SNCR) method. Both methods being utilized especially for NO.sub.x reduction. Typical NO.sub.x emissions from coal combustion, where staged air injection and SNCR, e.g. ammonia or urea injection, is utilized, are in the range of 50 to 60 ppm.
The SCR systems are being utilized to reduce NO.sub.x emissions from oil and gas firing applications, in which the initial NO.sub.x concentration in the flue gases can be much higher than in coal combustion, e.g. 500 to 1000 ppm. As the level of permitted NOX emission rates decreases may SCR also be utilized for further lowering the NOX emissions in pulverized combustion of solid fuels. In these systems the catalyst, e.g. V.sub.2 O.sub.5 /TiO.sub.2 or zeolite, is typically attached as a thin layer on a honeycomb type support structure. The flue gas flows through the passageways of the honeycomb elements or "cells" and NO is being reduced by e.g. ammonia in presence of the catalyst surface.
Such honeycomb elements need to be changed in certain intervals in order to replace the poisoned catalyst elements with new ones. The catalysts may be poisoned by various gaseous or ash components, e.g. alkali- or As-compounds. The dimensions of the passageways in such catalyst elements depend on how clean the gas to be cleaned is, i.e. free of dust and gaseous impurities. In gas and oil combustion, very dense gridded passageways can be used, whereas dusty gas from solid fuel combustion needs wider channels, and hence a large volume requirement for the catalyst honeycomb system.
It is e.g. known from European patent application EP 470659 to clean gas in a catalytic filter. The filter consists of a ceramic support and catalytic substance. The filter elements may be porous sintered elements in the form of filter candles. The filter elements are doped with a catalytic substance. Dust and gaseous organic compounds in hot gases can be removed with these catalytic filters simultaneously. A drawback in this gas cleaning system is that the entire catalyst filter elements have to be replaced by regenerated or new filter elements when either the filter part is made non-operative, by plugged pores, or the catalyst material is poisoned, by trace compounds, heavy metals, such as As, or alkaline deposition, or by some other reasons.
Multiple-passageway porous monolithic cross-flow filters are known from U.S. Pat. No. 4,781,831. These filters have a large amount of filtration surface area relative to the volume of the device and consequently provide a compact filtration device. The filtration device further utilizing effectively the multiple-passageways, provides a low pressure drop flow path for the filtrate. These monolithic filters can also be used as a membrane support for membrane devices. Such membranes can include separation barriers suitable for e.g. cross-flow microfiltration, ultrafiltration and gas separation. U.S. Pat. No. 4,983,423 shows a method for forming a porous membrane on such a membrane support.